Undesirable visible artifacts such as contour artifacts are sometimes present in a decompressed electronic image. Quantization during the compression of the image often causes losses in image information; hence the term “lossy compression”. Unfortunately, these losses may cause pixel-value errors during the decompression of the image, and these errors may cause visible artifacts in the decompressed image. Contour artifacts are line patterns that resemble the contour lines in a geographical map, and are typically more noticeable in the lower-intensity, i.e., darker, regions of the decompressed image than in the higher-intensity, i.e., brighter, regions of the image.
Unfortunately, some image-processing techniques such as gamma correction can increase the visibility of artifacts such as contour artifacts. Generally, gamma correction increases the contrast in the bright regions of an image relative to the contrast in the dark regions of the image. That is, brightness changes in the bright regions are amplified with respect to brightness changes in the dark regions. For example, a brightness change of 1 lumen in a bright region may be gamma corrected to 1.5 lumens, but a brightness change of 1 lumen in a dark region may be unchanged or gamma corrected to less than 1.5 lumens. The actual gamma-correction algorithm used depends on variables such as the characteristics of the image display device. Image processing circuits use gamma correction to compensate for the human eye's non-linear brightness response. The human eye is more sensitive to brightness changes in dark image regions than it is to brightness changes in bright image regions. That is, the human eye perceives a change of 1 lumen in a dark region as being a larger change than a change of 1 lumen in a bright region. Therefore, gamma correction allows the human eye to perceive a linear or nearly linear contrast scale across the entire brightness range of an image. But unfortunately, gamma correction often effectively increases the quantization levels in the dark regions relative to the quantization levels in the bright regions, and thus may increase the visibility of existing contour artifacts.